The present invention relates to a method and a plant for cutting by means of a laser beam using a laser device comprising a laser cutting head with a dual flow of assist gas and a double focus for focusing the laser beam.
In industry, it is well known to use a laser beam to cut or weld one or more metal workpieces. By way of example, documents DE-A-2713904, DE-A-4034745, JP-A-01048692, JP-A-56122690, WO 97/34730, JP-A-01005692, DE-A-4123716, JP-A-02030389, U.S. Pat. No. 4,871,897, JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693, JP-A-15694, JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553, DE-A-3619513 and DE-A-3934920 may be mentioned.
An alternative to the existing methods is proposed by document WO-A-98/14302 which teaches a method for cutting a metal workpiece using a transparent or reflecting optical means, that is to say a mirror or lens, in order to focus a laser beam onto several focusing points distinct one from the other and located on the same axis and having different convergence angles.
Moreover, it is known that cutting structural steels using oxygen or a gas mixture containing oxygen as an assist gas generates oxides on the cut faces, which prevents the cohesion of paint on workpieces cut in this way.
To try and solve this problem, there are currently two solutions.
The first solution consists in brushing the workpieces after each cutting, which is long and tedious.
The second solution is to cut the pieces under nitrogen at high pressure, typically of the order of 10 to 20 bar, which involves a high cost because of the considerable nitrogen consumption, the low cutting speeds and a limit on the thicknesses that can be cut.
Furthermore, in the case of stainless steels or coated aluminium, the usual presence of a plastic film on the surface, intended to protect them, often generates flash or prevents the correct execution of the cutting operation.
What is then involved is choosing a plastic coating suitable for laser cutting, but these are still rare at present and expensive, or to carry out a first vaporization pass, that is to say burning the surface of the plastic layer by a laser, before the actual cutting of the material, which complicates the procedure and is detrimental to productivity.
The aim of the present invention is thus to solve the aforementioned problems by providing an improved method of cutting or welding by means of a laser beam, that is to say a method which in particular allows structural steels to be cut while minimizing or removing the layer of oxides which is formed when the steels are cut under oxidizing gas and which allows stainless steels coated with a plastic film to be cut while avoiding carrying out a first vaporization pass before the actual cutting.
The invention therefore relates to a plant for cutting or welding by laser beam comprising:
at least one laser generator to generate at least one main laser beam,
a first optical means enabling the said main laser beam to be divided into at least two subsidiary laser beams,
at least one first outlet nozzle through which at least one of the said subsidiary laser beams passes,
at least a second outlet nozzle through which at least the other of the said subsidiary laser beams passes,
the said first and second outlet nozzles being approximately coaxial one with the other.
Depending on the case, the plant of the invention may comprise one or more of the following characteristics:
the central beam passes through the first outlet nozzle and the peripheral beam passes through the second outlet nozzle,
the first optical means and the second optical means are arranged coaxially one with respect to the other,
the said first optical means enables the main laser beam to be divided into a central beam and a peripheral beam of annular or cylindrical shape and the said central beam to be focused onto a focusing point, and moreover a second optical means enables the peripheral beam to be focused onto an annular focusing zone distinct from the focusing point,
the plant comprises a first source of assist gas feeding the first outlet nozzle and/or a second source of assist gas feeding the second outlet nozzle,
the first optical means and/or the second optical means are transparent or reflecting, preferably the first optical means and/or the second optical means are lenses.
The invention also relates to a method of cutting or welding at least one metal workpiece, in which:
(a) at least one main laser beam is generated,
(b) the said main laser beam is divided into at least two subsidiary laser beams,
(c) the said beam is focused onto a focusing point by means of a first focusing means,
(d) the said subsidiary beam is focused onto an annular focusing zone,
(e) the cutting or welding takes place by melting at least part of at least one workpiece by means of the said subsidiary beams.
According to another aspect, the invention also relates to a process for cutting or welding at least one metal workpiece, in which:
(a) at least one main laser beam is generated,
(b) the said main laser beam is divided into at least two subsidiary laser beams,
(c) one of the said subsidiary laser beams is guided through a first outlet nozzle and the said beam is focused onto a focusing point (PF) by means of a first focusing means,
(d) the other of the said subsidiary laser beams is guided through a second outlet nozzle arranged approximately coaxially with the first outlet nozzle and the said subsidiary beam is focused onto an annular focusing zone (ZAF) and,
e) the cutting or welding takes place by melting at least part of at least one workpiece by means of the said subsidiary beams.
Depending on the case, the method of the invention may comprise one or more of the following characteristics:
it comprises at least one of the additional steps of:
(f) feeding the first outlet nozzle and/or the second outlet nozzle with one or more gases or gas mixtures;
(g) carrying out a relative displacement movement of the workpiece or workpieces to be cut or welded with respect to the first outlet nozzle and/or the second outlet nozzle;
the subsidiary beam is focused onto the said annular focusing zone by means of a second focusing means.
In step (b), the main laser beam is divided by means of the first optical means into a central beam focusing onto the said focusing point and into a peripheral annular or cylindrical beam, guided or conveyed towards the second focusing means enabling the peripheral beam to be focused onto the annular focusing zone.
The central beam and the peripheral beam are guided or focused coaxially one with the other.
The central beam is focused onto a focusing point located near to the lower surface of the workpiece to be cut and the said focusing zone is focused close to the upper surface of the workpiece, in the thickness of the material or outside the latter, preferably the cutting gas used is a non-oxidizing gas.
The central beam is focused onto a focusing point located close to the upper surface of the workpiece to be cut and the said focusing zone is focused close to the upper surface of the workpiece, in the thickness of the material or outside the latter, preferably the cutting gas used is an oxidizing gas.
The first outlet nozzle is fed with an oxidizing gas and the second outlet nozzle is fed with a non-oxidizing gas, and a workpiece made of steel, aluminium or their alloys is cut.
The first outlet nozzle is fed with a non-oxidizing gas and the second outlet nozzle is fed with a non-oxidizing gas, and a workpiece made of steel coated with at least one polymer layer, preferably a stainless steel, is cut.
The non-oxidizing gas is nitrogen or a gas mixture containing mostly nitrogen and the oxidizing gas is oxygen or a gas mixture containing oxygen.
The relative position of at least one of two focusing means is adjusted or altered with respect to the other, in order to vary the focusing position or location of the focusing point and/or of the focusing zone in the thickness of the material to be cut.